Wikipedia, the online community encyclopedia, has also had to deal with such problems. The various Wikipedia sites that allow online users to add and edit content could open the door to potential malicious content, according to security experts.

That's almost what happened in November when a fake site masqueraded as a German version of Wikipedia hosting an entry on a variant of the MSBlast, or Blaster, computer worm. Instead, the web page attempted to compromise visitors' machines. While neither the site nor the content had been hosted on Wikimedia's servers, the phishing scam had such polish that it fooled at least one antivirus firm. The Wikimedia Foundation, the organisation that operates the Wikipedia sites, has seen the writing on the wall and taken steps to limit what users can do.

"We do not allow linking from executables," said Brad Patrick, general counsel for the Wikimedia Foundation. "The intent here is that we are always providing our readers with at least one additional step between us and any malicious content."

Even the virtual world of Second Life, which depends on user-created content to keep its economy going, has had to deal with virtual viruses, known as "grey goo."

For about about two hours on 19 November, the company that manages Second Life, Linden Lab, scrambled to contain an outbreak of Sonic the Hedgehog-esque gold rings. The objects spread within a region, slowing down the servers that maintain the Second Life world, or grid. It's the third major attack since September, each time the world has been overrun with quickly reproducing digital objects that have taken hours to clear out of the system.

"Specific to our own products and properties, including sites which host user generated content, we work constantly to prevent people from misusing our services to distribute malicious software," Barry Schnitt, spokesman for Google, stated in an email interview. "When we become aware of an instance where this happens, we take immediate action to limit user exposure."

That's good, but the companies need to attack the broad range of threats, rather than focus on, for example, child porn at the expense of malicious videos, said Websense's Hubbard.

"A lot of their security is geared towards child pornography, taking down content that they don't want on their site," Hubbard said. "They need to get more savvy and build up their security teams, because we are talking about hundreds of millions of pages changing daily."